How many layers and neurons on the layers does a neural network need for for sales prediction?
Every answer you get is just an opinion, it is based on experience.
My answer is a single hidden layer of 5 neurons.

Besides, I recommend you to use a TWEANN system (Topology and Weight Evolving ANN).

Such system applys genetic algorithm to search and optimize an ANN with the optimal topology and weights.

Take a look at NEAT and DXNN.